Searches were also being conducted at Navalny’s RosPil anti-corruption project and the home of Udaltsov’s parents, journalist Olga Romanova said late Monday.

The raids were linked to the violent clashes between police and protesters on May 6, when more than 400 people were detained in central Moscow, the government’s Investigative Committee said in a statement. Those demonstrations came the day before Putin was inaugurated.

Investigators instructed those targeted to report to Investigative Committee headquarters at 11 a.m. Tuesday, an hour before the planned start of an opposition march and rally dubbed “the March of Millions,” likely hindering their plans to participate.

Navalny posted comments and photos on the social networking site Twitter as police burst into his flat about 9 a.m.

"They are searching my house. It's about the mass disorder. They almost sawed down the door (in fact they did),” he wrote.

Social media have proved a powerful tool for opposition figures struggling to get their message across in the face of heavily censored state media.

Apart from Navalny's home, police searched the residences of former Deputy Prime Minister Boris Nemtsov and TV presenter-turned-opposition activist Ksenia Sobchak, whose late father, Anatoly Sobchak, was mayor of St. Petersburg in the 1990s and Putin’s mentor.

At a hastily organized news conference, activists discussed the raids as well as preparations for Tuesday’s demonstration, the first major street protest since Putin’s inauguration for a new six-year term.

“Due to today’s raids, we expect even more people to attend tomorrow’s rally,” Sergei Parkhomenko, a journalist and protest co-organizer, told journalists.

Udaltsov also attended the conference in Solidarity’s offices and said police confiscated six bags of his belongings, mostly containing electronics.

Lawyers suggested that the grounds for the aggressive search tactics were questionable.

“I see the searches as an act of intimidation. It was not essential to search the opposition leaders’ flats in the middle of the public-holiday period,” Valery Borshchyov, a lawyer and activist for the Helsinki Group, said in an interview.

“This is demonstrative on the back of the rally law increasing fines for demonstrators that was recently passed," he added. "The authorities want to send a signal to frighten society, to show that everyone who violates the new law will be tried.”

On Friday, Putin signed into law controversial new legislation stipulating fines of up to $10,000 for participants and $30,000 for organizers involved in protests that are unauthorized, attract larger numbers than permitted, or cause damage or injury.

Opposition politicians in the State Duma, the lower house of parliament, reacted angrily to the raids, issuing an open letter in which they compared them to the actions of the tsarist secret police on the eve of the 1917 October Revolution.

“We believe that this could provoke an irreversible rise in social tension and close the path to constructive evolution of Russia’s political system,” politicians from the opposition Just Russia party wrote.

Similar searches were reported at the property of an activist from the nationalist Other Russia movement on Sunday.

Some analysts interpreted the latest government moves as an attempt to avoid a repeat of the May 6 violence, in which protesters and police hurled stones and asphalt at one another.

“The aim of these searches was to prevent further mass disorder,” said Sergei Markov, a Kremlin adviser and vice president of the Plekhanov University of Economics, adding that the searches were “legitimate.”

But Andrei Kortunov, president of the New Eurasia Foundation, a social development agency, said in an interview that the raids also reflect the authorities’ desire to tighten their grip on society, which has recently experienced something of a political awakening.

“The electoral cycle brought with it heightened political consciousness," he said of protests that began late last year with disputed elections for parliament. "Now that the elections are over, authorities want to return to life as normal.”

But he said that the searches could further stoke the discontent. “Their actions may well lead to a radicalization of the protest movement,” he said.

Photo: Police officers standing guard Monday at the entrance of the Moscow building where opposition leader Alexei Navalny lives watch an opposition supporter who holds an iPad decorated with a picture of Navalny with the words "Because we exist." Credit: Maxim Shipenkov / European Pressphoto Agency.